The heated water evaporates but its steam does not go though the metal sheet - the heat accumulated at one side transfers to the clean water at the other side through thermal conduction of heat, without the two liquids mixing together (in a 'water bath' manner). Two independent deposits of water are needed because of sanitary issues - everything in contact with edibles needs to be cleaned regularly and follow certain strict health norms, so if the machine only had one deposit, the strict cleaning would certainly damage the resistance (the part which produces the heat). Also the resistance impedes or makes more difficult cleaning so two deposits are convenient also to facilitate cleaning. PS Just answering randomly, I'm a product engineer student and I'm currently analysing espresso machines as application of heat transference in products haha. Hope you got it.

So this is called a heat exchanger. The reason you have this is so you can have 2 different temps on one boiler. The blue > red is your brew water, 200F. The steam is 240+ much to hot to brew espresso but perfect for steaming milk. It allows the transfer of heat from one system to the next so you can reduce energy consumption. Just did a video on how espresso machines work on my page...

kzfaq.info/get/bejne/oct5i7l1p8eognU.html

Instablaster.

We flush them every year with citric acid. No dirtier than tap water...

you realize most residential building use copper pipes - because they are corrosion resistant and food safe ?

I break it down in my video

kzfaq.info/get/bejne/oct5i7l1p8eognU.html